This invention relates electromagnetic projectile launchers or railguns incorporating current guard plates to minimize railgun damage while maximizing projectile velocities.
Electromagnetic projectile launchers or railguns are of potential interest for military applications as a means for firing projectiles at high velocities. Conventional railguns involve a short duration launch of a high energy projectile. Common projectile energies are in the range of 1-20 megajoules and launch times are 1-10 milliseconds. This dictates that useful railguns operate as pulsed current devices, driven by pulsed power supplies. In order to maximize projectile acceleration, rail current densities must be very high.
When applying electromagnetic railgun technology to military applications, it becomes necessary to propel a large projectile, often as large as a 90 millimeter, 2 kilogram armor-penetrating shell, at supersonic velocities exceeding 2 km/s. However, in doing so, rapid rates of fire and railgun durability must be sustained. Typically, today's high energy railguns display unacceptable railgun damage during a single shot. Continuous-shot railguns, therefore, cannot be achieved. Both the projectile and the rails experience substantial melting and material vaporization. The result is that the railguns require new rails or rail honing after just a few shots.
As railgun development continues toward higher energy devices capable of firing larger projectiles, more progress must be made in two aspects of railgun design. First, railguns must be made capable of sustaining hundreds if not thousands of shots without any major heat related damage. Second, the railguns must be made capable of handling increased pulsed energies needed to propel massive projectiles at higher velocities without significant increases in rail damage caused by high local current densities. Both aspects are related in that they primarily represent direct effects of railgun current and its resulting current distribution in the rails.
Rail damage is principally caused by high current densities transferred from the current carrying rails to the sliding conductive armature or projectile. Most of the damage is heat-related. The sources of the heat-related damage are: 1) heat generated by the rail-armature interface contact voltage drop; 2) Joule heating from the current in the rails; and 3) friction heating. The first two sources of damage are strongly dependent on the local current density. Because the distribution of rail current naturally concentrates in the vicinity of sharp rail corners, previous railguns have attempted to reduce rail damage by designing rails with rounded corners. Unfortunately, designing rails with rounded corners has produced only limited success. Significant current densities and accompanying railgun damage still exist. Railguns capable of withstanding hundreds or thousands of shots have thus far not been produced using conventional rounded-rail techniques.
Some known railgun designs use augmenting conductors to increase the electromagnetic force placed on a projectile. Augmenting conductors increase the inductance gradient in the railgun bore thereby achieving comparable projectile acceleration forces at substantially reduced currents. The augmenting conductors are conductive elements placed external to, and parallel with, the external surfaces of the railgun rails. When coupling a power supply to both the rails and the augmenting conductors in series, projectile force and inductance gradient in a square or round-bore railgun can be increased as demonstrated in numerical examples summarized below in Table I.
TABLE I ______________________________________ Augmented Square and Round-Bore Railgun Performance Inductance Force Current Gradient (MN) (MA) (MH/m) ______________________________________ Square Bore 0.393 1.224 0.525 Augmented 0.543 0.961 1.175 Square-Bore Round-Bore 0.346 1.184 0.494 Augmented 0.316 0.710 1.252 Round-Bore ______________________________________
Results presented in Table I are for square and round-bore railguns shown in FIGS. 1, 3 and 5. In Table I, the square-bore railgun of FIG. 1 has an inside rail separation, b, of 4.0 cm, each rail has a height, h.sub.r, of 4.0 cm and a thickness, t, of 1.0 cm. When augmenting conductors are added, as shown in FIG. 3, each augmenting conductor is placed 2.0 mm outside the outer edge of each inside rail. Each augmenting conductor has 8.0 cm height, h.sub.g, and 1.0 cm thickness, t. Meanwhile round-bore railguns having an arcuate rail and an arcuate augmenting conductor is shown in FIG. 5. For the numerical examples of Table I, angle a is 45.degree., railbore diameter, r.sub.b, is 2.257 cm, rail and augmenting conductor thicknesses, t, are 1.0 cm and separation distance, d, is 2.0 mm. Table I figures for force, current, and inductance gradiant were derived using the aforementioned geometries for square-bore, augmented and non-augmented, railguns and round-bore, augmented and non-augmented railguns. The geometrical differences between square-bore and round-bore railguns are held constant so that the comparisons shown in Table I can be accurate.
While augmenting conductors increase projectile force, conventional augmented designs exacerbate current distribution problems. For a given quantity of total railgun current, the increased force obtainable from conventional augmented railguns incurs the liability of increased rail peak current densities along the inside corners of the rails. For continuous-shot railguns, the resulting rail damage would be unacceptable. Thus, the need arises to combine the effects of augmenting conductors with means of reducing local rail current densities usable in a continuous-shot railgun application.